Refuse a zero concurrency limit instead of reading it as the default

FLOW_MAX_WORKERS, FLOW_MAX_CASCADES and FLOW_MAX_RUNS are all pool sizes,
so 0 says neither "none" nor "unlimited" — it is a pool that cannot be
built. They are PositiveInt now, so a 0 fails at startup naming the
setting rather than being swallowed by `max_cascades or MAX_CASCADES`.
The consuming fallbacks take only None as "nobody said": explicit
`is None` in the executor, and no clamp on RunService.parallel.

docs/reference/configuration.md

@@ -135,7 +135,9 @@ warning into a refusal to start.
 
 The three concurrency limits are also flags on `fluksio serve` — `--max-workers`,
 `--max-cascades`, `--max-runs` — which outrank the file, and the engine says
-which numbers it started with in its first lines.
+which numbers it started with in its first lines. Each is a pool size, so each
+must be at least 1: a `0` is refused at startup by name rather than read as the
+default. Leave one empty (or unset) to get the default.
 
 An artifact is referred to by a run that recorded it or by a message currently
 holding it; anything else is what a camera published four hours ago, and the